what engine do you have?
please explain to me what happened..when did it first get hot..and,what all have you done
are you there?
how hot did it show on the gauge?
so,,you have already replaced the thermostat and the resivour ..corrrect?
you may just have air in the system now
lets bleed it this way
1,you need to remove the radiator/reservoir cap when engine is cold
2-start the engine and let it idle
3-turn the heater on high
4-keep coolant on hand and add it as needed...some coolant may gush out with the air,so be careful
5-keep doing this until the engine is at operating tempeture,the heater is hot and the coolant in the radiator settles down
6-then install the cap
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you are very welcome